1. Field of the Invention
The present invention relates to an image pickup apparatus equipped with an imaging element having a plurality of types of color filters and with a hue setting feature for processing image signals, an image pickup method, and a recording medium and a program therefor.
2. Description of the Related Art
The following will describe an example in which a user sets a hue in an image pickup apparatus. The example relates to a setting method for white balancing (hereinafter referred to as “WB”) in a digital camera.
In a conventional digital camera, the basic setting modes selected by a user to correct WB are an automatic white balancing (hereinafter referred to as “AWB”) mode and a manual mode. When the AWB mode is set, a light source or a color temperature is automatically determined to correct the WB accordingly. When the manual mode is set, a light source, such as an electric bulb or a fluorescent lamp, and a color temperature are specified to perform a predetermined WB. The manual mode is suited for a case where it is difficult to automatically determine a light source or a case where different color temperatures are to be set.
The aforementioned AWB will now be described. There are two techniques for accomplishing automatic WB according to color temperature changes. In a first technique, a region that should show an achromatic color is determined from image data (hereinafter referred to as “the determination of achromatic color”), then the WB is performed to make the determined region achromatic. According to a second technique, the image signals of the overall image data are averaged for each color component, and the mean values are used to accomplish the WB. To determine achromatic colors, a region that should look achromatic is defined as an achromatic color determining range on a predetermined hue plane, then the region that should show an achromatic color is determined by determining whether each region is within a defined chromatic color extraction range. The hue plane is a coordinate plane in which, for example, the axis of ordinate indicates the assessment values of changes in hue toward green from magenta via an achromatic color, while the axis of abscissa indicates the assessment values of changes in color temperature. The hue plane plots color temperatures and hues of diverse light sources. The hue plane will be explained in more detail hereinafter.
The first technique described above is effective when the region of a substance or object that should look achromatic (hereinafter referred to simply as “the range”) in image data in video signals obtained from an imaging element or the like is sufficiently large. This technique is disadvantageous, however, in that it is difficult to accurately perform WB if there is no region of a substance or object that should show an achromatic color or if the region is extremely small.
Even if a sufficiently large region of the substance that should look achromatic can be extracted, erroneous extraction of an achromatic color may result in the case of, for example, an achromatic color at a low color temperature and a flesh color at a high color temperature, or an achromatic color at a high color temperature and blue or the like at a low color temperature. In these cases, achromatic color extraction results that are similar to each other are provided. Especially when a wider achromatic color determining range is set, the AWB can be accomplished for a wider variety of light sources, while on the other hand, more determination errors may result in cases where similar achromatic color determination results are obtained.
For instance, an achromatic fluorescent lamp has more green components than a sunlight light source. Hence, for most fluorescent lamps, AWB is accomplished by making a setting to provide an extended tracing range toward green, i.e., an extended achromatic color determining range, on a hue plane. On the other hand, erroneous determination results in the case of a fluorescent lamp against lawn or the like of a pale color, thus preventing accurate WB adjustment. If, however, the tracing range is narrowed, then the types of light sources that can be covered by tracing will be inconveniently limited.
According to the second technique, the color components in image data are integrated, and the integrated value provides the achromatic color. This approach poses the following problem. When shooting a scene in which a particular color is dominant, e.g., a sunset scene, it is undesirably determined that the integrated value indicates an achromatic color, whereas the mean value determined by integration does not indicate an achromatic color. This results in a serious determination error.
There have been proposed a third technique that combines the first technique and the second technique discussed above, and a fourth technique in which the information regarding the brightness of an object is separately used to estimate the color temperature of a light source.
A proposed image pickup apparatus, for example, has a detector for detecting the brightness of an object, a limiter for limiting the white balancing range for at least an object of a low color temperature, and a balancing range changer for changing the limited white balancing range according to the outputs of the detector so as to expand the range when an object is dark. More specifically, if an object is relatively bright, then it is presumed that the user is likely to be outdoors and the white balancing for an outdoor sunlight light source is effected. Each time an object grows darker, cloudiness or shade is added to the AWB tracing range (achromatic color determining range), and the object grows further darker, then indoor light sources, such as electric bulbs and fluorescent lamps, are added to the AWB tracing range. Using these techniques makes it possible to automatically adjust white balance to obtain hues matched to human sense based on remembered/stored colors and/or experiences, according to shooting circumstances.
Although the above third and fourth techniques solve the problems with respect to the first and second techniques, to a certain extent, they are not fundamental solutions, because they are not capable of accurately determining light sources. More specifically, the techniques are effective within a range of light sources and environments that have been assumed beforehand, but proper WB cannot be accomplished when light sources and/or environments are out of the expected range. For instance, AWB may not be properly effected under an extremely bright bulb light source or when shooting a pale green object in a dark place.
There is a case where WB should fully trace the changes of light sources; on the other hand, there is a case where capturing the atmosphere of a scene is more important than making WB fully trace light source changes. The selection between these two cases depends on the taste or intention of a shooter. For example, a depiction with a remaining reddish tint and a depiction with full WB effected under a bulb light source are both frequently used, depending on shooting scenes. It is therefore difficult to decide on an optimum tracing range of expected light sources and the optimum tracing range also depends on the shooter'intention. This poses a problem in that it is extremely difficult to determine standard optimum WB settings for multiple users. Furthermore, conventionally it has been impossible for users to visually recognize the WB settings in terms of colors.
When setting a color shade or a hue of an image signal, in an RGB space, for example, a user cannot visually identify RGB set values in setting each of the RGB independent parameters. It is therefore difficult to determine optimum color balance. Furthermore, when checking the color shade of an image to be shot displayed in a histogram, the entire color balance cannot be visually perceived by merely observing the respective independent levels of the RGB.